I know there is the official dog tag looking thing, and I just bought a pack of them and plan to use the official kind myself. I did notice, however, that "moun10bike"s coins are in the regular index as travel bugs.
Since travel bugs are apparently tracked by serial number, someone would have to assign a serial number to a bug issued by anyone other than geocaching.com itself.
Question I have then is - what other than the official dogtag is in the system as a legitimate indexed bug? Obviously moun10bike's coins are. How about the texas or canadian coins? Anything else? Just curious.